The present invention relates to a capacitance pressure transducer to measure the pressure of a gas.
Capacitance pressure transducers are constructed around a plane parallel capacitor plate formed by a diaphragm (normally a metal membrane) supported by a diaphragm tension support ring, herinafter referred to as a "tension ring" and an electrode disk (normally ceramic deposited with a conductive material). The diaphragm deflects relative to the electrode disk under application of the gas pressure to in turn produce a capacitance having a direct correlation to the gas pressure. The capacitance is electronically converted into a read out measurement of the gas pressure.
The plane parallel plate capacitor is housed to form a cell. The cell is formed by a cell housing that contains the plane parallel plate capacitor and an inlet tube through which the gas to be measured enters the cell housing. Conventionally, the cell housing is provided with opposed end covers that are welded to the tension ring at or near the vicinity of the diaphragm attachment point. The electrode disk is held in position by a spring against reference surfaces provided on the tension ring. The mounting of the electrode disk allows for differential thermal expansion between the tension ring and the electrode disk while maintaining the proper positioning of the electrode disk relative to the diaphragm. A baffle formed by a plate or wire mesh screen is positioned at the diaphragm end of the inlet tube. The baffle acts as a heat radiation shield for the diaphragm. The baffle has a secondary purpose of protecting the diaphragm from objects which may fall down the inlet tube.
In prior art capacitance pressure transducers, an external enclosure is provided to mechanically protect the cell and also to mount the electrical components used in performing the electronic gas pressure measurement. Both the cell housing and the inlet tube are connected to the external enclosure. Additionally, a heated inner enclosure of thin drawn configuration can be provided within the external enclosure to house the cell housing. The cell housing and components contained within the cell housing are maintained within the heated inner enclosure at a nearly constant temperature, above the ambient temperature, to surpress environmental temperature changes from producing thermal induced changes in the pressure responsive elements contained within the cell housing.
It has been found by the inventor herein that several factors related to the design of prior art capacitance pressure transducers act to degrade performance characteristics of such prior art transducers. As will be discussed, these factors have been isolated in the present invention to provide a capacitance pressure transducer having improved performance characteristics over the prior art.